Dealing with classical structural scan test, it is to be avoided as it allows for the downloading of all the circuit internal registers. Therefore, using such a technique would allow hackers to access the contents of the secret sub-module. Concerning functional application-like tests, the circuit is being sent stimuli as it would be receiving in its end application. In this case, the test patterns contain information about “how to use the secret module”, and about “what are the expected results”. Therefore at least part of its secret functionality (the secret keys) is revealed. The test operation is thus not confidential.
US patent application US 2002/0069387 describes a sub-module for example of a low confidentiality level, tested by a deterministic logic built-in self test (DLBIST) circuit. Such self-testing solutions are useful because self-testing the sub-module allows a reduction of the technical requirements necessary for performing external tests on a complex integrated circuit. Yet, when such a circuit is scanned by a conventional structural scan test, information would still be revealed on the nature of the sub-module.